Hey, I have been riding for about 14 years. I train ponies and horses all the time, but recently, whenever I ride a horse for more than a half hour, my back starts to kill me. It never happens with ponies and I'm only 21 years old. My trainer says I'm not doing anything wrong and that I ride horses and ponies very similarly position wise.
Do I need more muscle in my back? It's nothing medical, I already checked that out.